Measurement of the mechanical behavior of the lungs of anesthetized guinea pigs is used as an assay of response to irritant air pollutants. Work is in progress on the effects of sulfuric acid mist of different particle sizes. The problem of the joint effect of sulfuric acid and sulfur dioxide at low concentrations is the next planned step in these studies. We are preparing to study the acute effects of low levels of nitrogen dioxide with the intention of proceeding later on to examine the joint effects of nitrogen dioxide and sulfur dioxide and the possible effects of particulate material on the response to nitrogen dioxide. The existing data published in both these areas is somewhat conflicting.